The present invention relates generally to illumination apparatuses, and more particularly to an illumination apparatus for illuminating a reticle or a mask (these terms are used interchangeably in the present application) on which a pattern is drawn, in an exposure apparatus for use with a photolithography process for fabricating semiconductor devices, liquid crystal display devices, image pick-up devices (CCD, and the like), thin-film magnetic heads, and the like.
Recent fine and highly integrated circuit patterns have required wafers with a critical dimension of 0.3 μm or less, and accordingly the lithography process has mainly used a projection exposure apparatus so as to meet this requirement. Generally, a projection exposure apparatus includes an illumination optical system for illuminating a mask, and a projection optical system, located between the mask and an object to be exposed, for projecting a pattern on the illuminated mask onto the object to be exposed. The illumination optical system introduces light from a light source to an optical integrator such as a fly-eye lens, and the like, and uses its exit plane as a secondary light source, thus Koehler-illuminates the plane of the mask via a condenser lens.
The circuit pattern may be classified into a line and space pattern as adjacent periodic linear patterns, a contact hole pattern as adjacent periodic hole-shaped patterns, and an isolated pattern that is not adjacent but is considered to be isolated. A pattern transfer with high resolution requires a selection of optimal exposure condition (such as an illumination condition and an exposure dose) in accordance with a kind of the pattern. The exposure condition includes, for example, an illumination condition and exposure amount, and the illumination condition includes the coherency σ. The coherency σ is the numerical aperture (NA) at the mask side of the illumination optical system as opposed to the NA at the mask side of the projection optical system, and the exposure optical system usually uses a partially coherent illumination of 0<σ<1.
A phase shift mask with the relatively small σ with a phase shift mask, and a binary mask with modified illumination are suitable to the line and space pattern. The modified illumination is also called an off-axis illumination or oblique incidence illumination, and is a technique to turn an effective light source distribution into an annular or quadrupole shape, thus illuminating a mask using an obliquely incident beam. On the other hand, a half-tone or binary mask combined with a relatively large σ is suitable to the contact hole and isolated patterns.
A method has already been proposed which changes the coherency σ for the entire mask pattern according to mask patterns, for example, by making switchable a plurality of aperture stops, such as circular or annular aperture stop, which is usually provided just after the optical integrator in the illumination optical system.
The recent semiconductor industry has shifted its production from a memory chip, such as conventional DRAM, to a highly value-added system chip that integrates memory and logic patterns in a single chip. A memory pattern has a high pattern density and mainly a line and space pattern, and thus is congenial with a relatively small σ or an annular illumination. On the other hand, a logic pattern has a low pattern density and mainly an isolated pattern, and thus is congenial with an illumination with the relatively large σ. A fabrication of a system chip requires a blend of memory and logic patterns in a mask pattern.
However, the conventional configuration cannot partially modify the illumination condition for the mask plane. As a consequence, the mask having two kinds of patterns should be illuminated under a single illumination condition congenial with only one of these patterns, and thus the optimal lithography process and desired resolution have been hard to be obtained